1. Field of the Invention
This invention pertains to a cushioning system for footwear that enhances dynamic stability. More particularly, this invention pertains to compressible and expandable bladders extending along a portion of the sole and wrapping upward to embrace a portion of the foot for dynamically providing foot stability upon loading in shoes.
2. Description of Related Art
Shoe design reflects a highly refined combination of elements that cooperatively interact to minimize shoe weight while maximizing comfort, cushioning, stability and durability. However, these objectives must be balanced to avoid potential conflict with each other. Efforts to achieve one of the objectives can have deleterious effect on one or more of the others. As a result, the shoe industry has invested significantly in the study of human anatomy and biomechanics in its continuing efforts to optimize these competing objectives. Efforts have in a large part been directed at optimizing qualities of cushioning and stability.
Athletic shoes are of particular interest because they are subject to repetitive compression with high loads associated from running or jumping, which ultimately deteriorate the shoe materials. Yet, over the life of the shoe, the shoe must continue to provide cushioning and stability. Stability is the objective that is concerned with maintaining a wearer's foot in a fixed position within the shoe and preventing the shoe from rolling over a lateral or medial side edge of the shoe. Maintaining stability for the duration of the shoe is particularly important for preserving a healthy foot.
Shoe designs that focus on optimizing stability ultimately reduce risks of injury. A common such injury is sideways (i.e., lateral or medial) foot rotation. Sports such as basketball, tennis, indoor and outdoor soccer, rugby, lacrosse, and football as well as a wide range of other activities require frequent and quick lateral bodily movements. A secure foot plant becomes essential to the movement of the upper portion of the body. Injury often occurs when the foot plant is not secure and stable. For example, a significant ankle injury can occur when the foot rotates sideways over the edge of a shoe. This sideways rotation can over-extend any inherent flexibility of the ankle joint. Rotation of the foot outward towards a lateral side of the foot can result in pulled tendons or a sprained or broken ankle, and foot rotation inward toward a medial side of the foot can have like detrimental consequences.
A shoe typically comprises a multiple part construction. Generally, a shoe may be divided into four sections. An “outsole”, often called a “ground engaging surface,” is located on the bottom of a shoe. An “upper” is the top portion of the shoe that encircles or envelopes a user's foot. Inside of the upper can be an insole, which is typically a pad-like member directly under a user's foot. Finally, there is a “midsole” positioned between the outsole and the upper. A footbed for a wearer's foot to rest on can be either the top surface of the insole or a top surface of the midsole.
The outsole is generally formed of a durable material for resisting wear during use; typically the material is rubber or a rubber-like composite. The material selections for the upper are numerous, for example, leather, polymers, a variety of natural or synthetic webs or meshes, and materials that are breatheable, water resistant, water repellant may be chosen for their appearance and/or performance.
The midsole that forms a middle surface of the shoe is typically comprised of cushioning material. The cushioning material traditionally included polyurethane or ethylene vinyl acetate (“EVA”) foam. However, from about 1970, manufacturers began focusing their attention upon enhancing the midsole cushioning in shoes, especially for athletic shoes. These types of shoes are subject to intense compressions in addition to a greater numbers of compression cycles over the life of the shoe. The use of resilient bladders combined with traditional cushioning materials represented a marked improvement in midsole design. In particular, the use of resilient, inflated bladder midsole inserts, e.g., in accordance with the teachings of U.S. Pat. Nos. 4,183,156, 4,219,945, and 4,340,626 to Rudy, provided longevity to the midsole cushioning. The industry's focus on improving cushioning greatly advanced the state of the art in shoe design. In some cases, however, the benefits realized by cushioning were offset by a degradation of side-to-side shoe stability in response to lateral or medial movements and loads.
U.S. Pat. No. 5,425,184 to Lyden et al., discusses shoe progression and, in particular, evolutionary increases in midsole height. Shoe midsoles have increased in thickness largely to address the cushioning aspect of shoe design. These height increases have causes some stability problems. Lyden '184 addresses a height problem in the heel region where the forward foot motion from a heel strike advancing to a toe push off is rotated with an undesirable velocity due to the larger height of the heel region creating a lever arm and a greater moment propelling the foot forward.
The increase in midsole thickness creates a specific stability problem in activities where frequent and firm foot plants and quick lateral bodily movements are common. Specifically, the problem is that there is a tendency for detrimental sideways foot rotation over a side edge of the shoe.
Foot rotation in the sideways direction can be envisioned by picturing foot rotation about an imaginary line that extends generally longitudinally for the length of the foot, from the middle of the ankle to the middle of the toes. The tendency for rotation of the foot about this line is accentuated by increasing the distance between the bottom of the foot and the ground surface. Foot rotation about this longitudinal line, and consequently foot rotation sideways over the edge of the shoe, is most commonly and most dramatically noted in high-heeled women's shoes. Sideways rolling-over is due in part to the great distance between the foot and ground. The greater the distance, the easier it is to rotate sideways over the edge of the shoe. While most athletic shoes do not reach the height of women's high-heeled shoes, the lateral stability demand of athletic shoes is just as great if not greater. Lateral stability is essential for frequent and firm foot plants and quick lateral bodily movements necessary in sports.
What is needed is a stability device that prevents undesirable sideways foot rotation, increases security of the foot within the shoe, and facilitates keeping the foot in position on the footbed of the shoe, yet remains flexible and cushions the foot.